Optical information media such as a read-only optical disk and recordable optical disk are recently significantly used as an information recording medium recording and preserving a large amount of information such as a motion picture information. For example, a compact disk among these media generally has a structure in which a reflection layer is formed of a metal thin film on a transparent substrate made of a polycarbonate on which fine grooves, embossed pit trains have been formed, and an organic protective layer having a thickness of about 5 to 20 μm is provided for preventing deterioration of this reflective layer.
On the other hand, for enhancing recording capacity, optical disks obtained by bonding two transparent substrates as described for example in Japanese Patent Application Laid-Open (JP-A) No. 08-212597, namely, DVDs (digital video disk or digital versatile disk) are widely spreading, recently.
Further, as a high density type optical disk capable of enhancing recording capacity further than DVD and recording high quality motion picture information for a long period of time, optical information media described in JP-A No. 08-235638 are suggested. This high density type optical disk is an optical information medium obtained by forming a recording layer on a transparent or opaque supporting substrate formed of plastic, then, laminating a light transmission layer having a thickness of about 100 μm on the recording layer, which is used by being irradiated with recording light and/or reading light through the light transmission layer.
Regarding a high density type optical disk, there is suggested further increase in density by short wavelength recording utilizing laser light having wavelength shorter than red laser, for example, having a wavelength of 400 nm, used for recording and/or reading of conventional optical information media such as a compact disk and DVD, as described in JP-A No. 11-273147.
As a method of forming a light transmission layer, there are listed, for example, (1) a method of bonding a film of a thermoplastic material such as a polycarbonate or a glass plate on a recording layer using an ultraviolet-curable resin, (2) a method of applying an ultraviolet-curable resin on a recording layer by a spin coat method, then, irradiating with ultraviolet to obtain a light transmission layer, as described in JP-A No. 08-235638.
When a thermoplastic transparent plastic film is used in the above-mentioned light transmission layer forming method (1) described in JP-A No. 08-235638, there is a problem of deterioration in its scratch resistance, and when a glass plate is pasted, there is a problem that weight is large and handling is inferior though scratch resistance is superior. There is also a problem that a light transmission layer made of glass cracks easily by impact from outside.
As ultraviolet-curable resins usable in the above-mentioned light transmission layer forming method (2), there are mentioned ultraviolet-curable resins described in JP-A No. 03-131605. However, a light transmission layer is formed using this curable resin, its scratch resistance is more excellent than that of a transparent film made of plastic as described above, however, since transmittance for light having a wavelength of about 400 nm is low, short wavelength recording is difficult, and since the curable resin shows high volume shrinkage in curing, significant skew occurs on an optical disk, these being practical problems.